Method of making an electron emitter



Feb. 10, 1959 b. DOBISQHEK ETAL 2,873,218

METHOD OF MAKING AN ELECTRON EMITTER Filed April 16, 1957 FIG.2

INVENTOR. DIETRICH DOBISCHEK By JQHN A. SCHWEITZER PERRY T. WARD 5 W ATTOR/VE United States Patent METHOD OF MAKING AN ELECTRON EMITTER Dietrich Dobischek and John A. Schweitzer, Asbury Park, N. 3., and Perry T. Ward, Chicago, 11]., assignors to the United States of America as represented by the Secretary of the Army Application April 16, 1957, Serial No. 653,259

2 Claims. (Cl. 111-222 (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The invention relates to cold cathode emitters capable of self-sustained emission without the use of heaters and useful electron discharge devices, and more particularly to improvements in coatings for such emitters, and to methods of preparing same. One type of such emitter is disclosed and claimed in the copending application of Dietrich Dobischek et al., Serial No. 408,053, filed February 3, 1954, now Patent No. 2,802,127. Another type of such emitter is disclosed and claimed in the copending application of Dietrich Dobischek et al., Serial No. 568,- 941, filed March 1, 1956, now Patent No. 2,842,706.

It is an object of this invention to provide an improved emitter of this type which can be exposed to air during the preliminary part of its fabrication and later assembled with other electrodes in an envelope and subjected to a final forming process.

For a more detailed description of the invention together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawing, wherein:

Figure 1 shows a partially broken away perspective view of a cathode made in accordance with the invention; and

Figure 2 shows a cross-sectional view of a cathode in accordance with another embodiment of the invention.

Referring to the drawing, Figure 1 shows in greatly enlarged cross-section the cathode comprising a metal core body or base 12 and a layer 14 of magnesium oxide, deposited on all or part of the body 12.

In carrying out the process, the metal base 12 is first roughened. A roughened surface may be produced mechanically by a sandblasting treatment or by any method known in the art such as etching. The metal core 12 may be zirconium, nickel, copper or other suitable material. It has been found that zirconium provides a cathode having less noise emission. The core body 12 may be cylindrical or flat, or have other shape.

The next step is to deposit in open air a highly porous coating of magnesium oxide on the roughened metal base 12 by spraying the latter with any of the following mixtures:

(a) Any organic compound which forms magnesium oxide by decomposition, such as magnesium oxalate suspended in amyl acetate.

(b) A suspension of magnesium oxide light in a suitable liquid carrier such as amyl acetate in the proportion of 2-10 grams magnesium oxide light to 100 milliliters amyl acetate.

(0) A suspension comprising a combination of (a) and (b) above. This suspension may be made up from 10 grams magnesium oxide light, 10 grams magnesium oxalate and 300 milliliters amyl acetate.

The above coating may also be applied by dripping or painting the suspensions on to the metal base 12, which may be slightly heated before coating and between each coating so that the suspension would not run and cause uneven coatings. The dried coating is stable in air and is particularly resistant to contamination during handling. For extra precaution it may be stored in a desiccator until mounted and sealed in a tube envelope.

The thickness of the coating should be thick enough to avoid deterioration that may result from arcing. It

should, however, not be so thick that it will become flaky. A thickness which has been found suitable is about 10* centimeters.

The coated cathode together with other electrodes required for the finished electron tube is next mounted in a tube envelope which is exhausted to a low pressure of approximately 10- to 10- millimeters of mercury and baked for one to two hours are temperature which is below the softening point of the glass used. Temperatures of 250-500 degrees centigrade have been used. This at least partially decomposes the magnesium oxalate into MgO and C0 The MgO remains as a highly porous layer on the metal base.

To complete the decomposition of the oxalate and activate the magnesium oxide layer so that self-sustained emission is obtained, the coated cathode 10 is next heated for about five minutes in a firing atmosphere of air at a pressure of 150 millimeters of mercury and at a temperature of about 800 C. To avoid softening of the glass, the cathode can be heated by radio frequency induction methods.

Experimentally, it was found that self-sustained emission could also be obtained when inert gas, oxygen or a vacuum was used instead of air as the firing environment, but best results were obtained with air.

The tube is degassed by firing in vacuum for 5 minutes at about fill-800 C. A conventional getter installed in the tube is then flashed for the usual purpose, and the tube is tipped ofi at a pressure of about 10- millimeters of mercury. The tube is now ready for operation.

A modified form of the invention which has proved to be of good practical value is shown in Figure 2. In this one, the cathode 20 comprises a metal core body 22, a layer 24 of pure magnesium, and a layer 26 of magnesium oxide superimposed thereon. The use of a pure magnesium layer is of advantage where magnesium oxide alone is used to form the layer. Where magnesium oxalate is used the pure magnesium layer is not necessary. The thickness of the magnesium layer should be about 500 angstroms.

Although the layers 24 and 26 are shown in Figure 2 as discrete layers, they are not strictly separate layers but are combined and fused together by the baking op eration to promote an activating reaction between the pure magnesium film 24 and the magnesium oxide layer 26.

By incorporating more electrodes in the tube, it can be used for the same purposes as ordinary electron tubes, such as triodes, pentodes and the like.

With the above process an extremely smooth and homogeneous layer can be made by the use of very fine particles of magnesium oxide, and results in a cathode with a minimum of noise. Rougher and less homogeneous coatings will produce a greater amount of noise and can be used where the tube is to be used as a noise generator.

It has been found experimentally that cathodes of this type are capable of self-sustained electron emission of as high as milliamperes per square centimeter in vacuum without heating the cathode. To produce the effect, a collector electrode is placed in front of the coated cathode and is made 200 v. to 300 v. positive with respect to the metal base of the coated cathode.

Upon charging the film surface positively to approximately the collector potential, a high electric field is established across the thin magnesium oxide layer, and under the influence of this field, electrons are emitted persistently from the magnesium oxide layer. By varying the collector voltage the electron current can be varied from a few microamperes to 100 milliamperes, changing exponentially with collector voltage.

While there have been described what are at present believed to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, there fore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. The method of making an electron emitter comprising the steps of depositing upon a metal base a coating including a suspension of magnesium oxide light in amyl acetate in the proportion of 2-10 grams magnesium oxide light to 100 milliliters amyl acetate, heating said coating to remove said amyl acetate thereby leaving on said metal base a highly porous coating of magnesium oxide, and activating said emitter by heating it for about five minutes at a temperature of about 800 C. in an atmosphere of air at a pressure of about 150 millimeters of mercury.

2. The method of making an electron emitter comprising the steps of depositing upon a metal base a layer comprising a suspension made up of 10 grams magnesium oxide light, 10 grams magnesium oxalate and 300 milliliters amyl acetate which can be decomposed by heat to form a highly porous layer of magnesium oxide, and heating said layer for about 5 minutes at a temperature of about 800 C. in an atmosphere of air at a pressure of about 150 millimeters of mercury, thereby decompos ing said magnesium oxalate to leave a highly porous layer of magnesium oxide and activating said layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,620,287 Brarnley Dec. 2, 1952 2,708,726 Atherton May 17, 1955 2,796,364 Suchotf June 18, 1957 FOREIGN PATENTS 503,112 Canada May 25, 1954 

1. THE METHOD OF MAKING AN ELECTRON EMITTER COMPRISING THE STEPS OF DEPOSITING UPON A METAL BASE A COATING INCLUDING A SUSPENSION OF MAGNESIUM OXIDE LIGHT IN AMYL ACETATE IN THE PROPORTION OF 2-10 GRAMS MAGNESIUM OXIDE LIGHT TO 100 MILLILITRERS AMYL ACETATE, HEATING SAID COATING TO REMOVE SAID AMYL ACETATE THEREBY LEAVING ON SAIDE METAL BASE A HIGHLY POROUS COATING OF MAGNESIUM OXIDE AND ACTIVATING SAID EMITTER BY HEATING IF FOR ABOUT FIVE MINUTES AT A TEMPERTAURE OF ABOUT 800* C. IN AN ATMOSPHERE AT AIR AT A PRESSURE OF ABOUT 15 MILLIMETERS OF MERCURY. 